An unusual and unique eukaryotic transcriptional control element which markedly increases the transcriptional activity of eukaryotic genes is the activator or enhancer element. We first characterized the enhancer elements in the genomes of simian virus 40 (SV40) and the Moloney sarcoma virus (MSV) and demonstrated that these sequences act in a host-cell specific fashion. This implies a potential for tissue-specific gene expression. The core activity of the enhancer has been localized to a very short stretch of nucleotides within the SV40 72 bp repeat. In attempting to identify cellular enhancer elements, we have investigated sequences in the putative control regions (the long terminal repeats or LTRs) of endogenous retroviruses. Certain of these cloned sequences appear to manifest low levels of enhancer activity.